1) Field of the Invention
The present invention relates to an optical, encoder, a motor driver, and an image forming apparatus. Particularly, the present invention relates to a motor driver that controls a driving of a belt member such as a photosensitive belt and an intermediate transfer belt, and an image forming apparatus such as a copying machine, a printer, and a facsimile having this motor driver.
2) Description of the Related Art
In an image forming apparatus, a drive control device, which controls a driving of a belt member that is utilized to form an image, such as a photosensitive belt, an intermediate transfer belt, and a paper transfer belt, is installed. In order to control the driving of the belt member to form an image, it is necessary to carry out a positioning of an image in high precision on the surface of the belt member or on the surface of a recording member that is conveyed by the belt member. In other words, in the image forming apparatus, the precision of a moving quantity of the belt member per unit time and the precision of a position (moving position) at a predetermined point on the belt member at a predetermined time give a large influence to the quality of the image formed. Therefore, in such a drive control device, it is required to control in high precision the moving quantity of the belt member per unit time and the moving position of the belt member at a predetermined time. However, the moving speed of the belt member can easily change due to various factors such as a change in the load of a member that is brought into contact with the belt member, and therefore, it is extremely difficult to entirely eliminate the change in the speed of the belt member. As a result, in such a drive control device, it is difficult to control in high precision the moving quantity of the belt member per unit time and the moving position of the belt member at a predetermined time.
Japanese Patent Application Laid Open No. H9-114348 discloses a drive control device that has marks formed on the front surface or the back surface of an endless belt member, and when a sensor detects the marks, the drive control device feeds back a result of the detection to a drive control. Specifically, a plurality of marks is continuously formed on the belt member at equal intervals on the surface moving direction of the belt member such as a recording paper conveyer belt. A mark detector detects these marks. This device directly observes a movement of the belt member itself. Therefore, this device can carry out the drive control in higher precision than that of a device that carries out the drive control based on a rotary angle speed of a supporting roller that supports the belt member.
In various kinds of image forming apparatuses, it is highly necessary to precisely control a move or a displacement of moving members that are included in the device. For example, in a digital color copying apparatus, runs of a latent image carrier that is formed in a drum shape, an intermediate transfer belt that is used to transfer a toner image, and a sheet conveyer belt that conveys transfer paper respectively need to be controlled in high precision. Therefore, the run control with an encoder is essential.
In encoders, generally, a main scale is provided on the running surface of a shifter. An index scale is disposed close to this main scale. Light from a light source is emitted onto the main scale. A light receiver receives, through the index scale, the light that is reflected from the main scale or the light that is transmitted through the main scale. The run of the shifter is detected by utilizing a change in the intensity of the received light following a relative positional displacement between the main scale and the index scale following the running of the shifter.
As an example application of the run control of a shifter using the encoder to the image forming apparatus, the following method is known. Marks are formed on the surface of a belt as a shifter. A sensor detects the marks, calculates a belt surface speed based on a pulse interval obtained, and feeds back the belt surface speed to the control (for example, see Japanese Patent Application Laid Open Publications No. H6-263281 and No. H9-114348).
According to this method of feedback, as the movement of the belt surface can be observed directly, a move quantity of the belt surface can be controlled directly.
In general, it is extremely difficult to form a belt member in a uniform thickness in the belt moving direction. The thickness of the belt member changes when the belt member is deformed due to a tension applied to the belt during the move of the belt. Therefore, during the movement of the belt member, an interval between the marks formed on the belt member and the mark detector changes. At the time of detecting the marks at a belt portion between a plurality of supporting members that support the belt member, the interval between the marks and the mark detector changes when the belt portion vibrates. As explained above, when the interval changes, the distance (a detection distance) between the mark detector and the marks changes at each detection timing. Consequently, a detection error occurs when the mark detector optically detects the marks. This problem will be explained in more detail below.
FIG. 29 is a schematic configuration diagram of a belt drive unit that drives a belt member 560. This belt drive unit includes a belt driving motor 581 as a driving force transmitter that generates a driving force to drive a driving roller 562 as a supporting member to support the belt member 560, and a decelerator 584. When the driving force is transmitted from the belt driving motor 581 to the driving roller 562 via the decelerator 584, the belt member 560 moves to a direction shown in the drawing. This belt drive unit also includes a mark sensor 590 as a mark detector that detects mark holes 585 provided on the belt member 560. These marks 585 provided on the belt member 560 consist of a plurality of through-holes that continue at a constant interval in a belt moving direction. The mark sensor 590 includes a transmission-type photo interrupter that has a light emitter and a light receiver disposed oppositely.
FIGS. 30A and 30B are enlarged views of a portion of the belt member 560 that faces the mark sensor 590. FIGS. 30C and 30D are graphs illustrating an output waveform of the mark sensor 590 corresponding to FIGS. 30A and 30B respectively. When the mark holes 585 move following the movement of the belt member 560, the light emitted from a light emitting element 591 of the mark sensor 590 is transmitted through only the mark hole portions. A light receiving element 592 intermittently receives the transmitted light. Therefore, the mark sensor 590 forms output waveforms as shown in FIGS. 30C and 30D. Conventionally, the light emitting element 591 that is used in the mark sensor 590 emits light in a radial shape with a center of the light around the light emitting element 591.
Therefore, for example, when a detection distance L1 between the light emitting element 591 and the belt surface shown in FIG. 30A changes to a detection distance L2 shown in FIG. 30B, a cross-sectional length of the light emitted on a virtual plane C including the light receiving surface of the light receiving element 592 becomes large in the belt moving direction. Consequently, the light receiving element 592 starts receiving the light that is transmitted through one mark hole 585, at an early timing. Further, the light receiving element 592 ends receiving the light at a late timing.
Therefore, when the mark interval is small, before ending the reception of the light, the light receiving element 592 starts receiving the light that is transmitted through the next mark hole 585. In this case, the output form of the mark sensor 590 has a small difference between a low level and a high level as shown in FIG. 30D. When the mark interval is sufficiently large, the difference between the low level and the high level can be made large even when the detection distance changes as described above. However, as the drive control of the belt member for image formation is required to have high precision, it is also important that a sampling interval for detecting a mark is small as far as possible. Therefore, the interval between the marks needs to be small, and the output waveform as shown in FIG. 30D is obtained. Based on the output waveform, for example, when the output is pulsed at a certain threshold value, a duty ratio of the pulse changes. In this case, when the marks are detected at a leading time or a trailing time of the pulse, an error occurs in the mark detection timing.
A detailed error will be examined by taking the following assumption. The mark holes 585 are formed in 1 millimeter L/S (line and space) in the belt moving direction. The interval between the light emitting element 591 and the mark hole 585, and the interval between the light receiving element 592 and the mark interval 585 are the same of 1 millimeter (the detection distance L1=1 millimeter in FIG. 30A). The end of the mark hole 585 in the belt moving direction is on a virtual line that connects between the center of the light emitting element 591 and the center of the light receiving element 592. In this case, a divergence angle of the light that is transmitted through the mark hole 585 becomes 45 degrees. Therefore, the cross-sectional length of the light on the virtual plane C in the belt moving direction shown in FIG. 30A becomes 2 millimeters. In this state, when the mark hole 585 moves toward the light emitting element 591 by only 0.5 millimeter (the detection distance L2=0.5 millimeter in FIG. 30B), the divergence angle becomes 60 degrees. Consequently, the cross-sectional length of the light on the virtual plane C in the belt moving direction shown in FIG. 30B becomes 4 millimeters. Therefore, when the end of the mark hole 585 in the belt moving direction is on the virtual line that connects between the center of the light emitting element 591 and the center of the light receiving element 592, an error of 1 millimeter already occurs. In the image forming apparatus of 600 dots per inch, the line interval of an image corresponding to a belt moving direction is 42 micrometers. In order to increase the positional precision of the line of each image, the mark detection interval also needs to be decreased corresponding to this short line interval. Therefore, the error of this 1 millimeter is extremely large. In the configuration shown in FIG. 29, a deviation in the detection distance of the 0.5 millimeters could occur sufficiently. Even when the deviation in the detection distance is controlled with a known tool that stabilizes the belt run, a deviation of about 0.1 millimeter cannot be avoided.
In the above explanation, a transmission-type mark sensor is taken up as an example. Therefore, the vibration of the belt member affects large the detection error, and the deviation in the thickness of the belt member does not substantially affect the detection error. However, the detection error similarly occurs in a reflection-type mark sensor. In this case, in addition to the vibration of the belt member, the deviation in the thickness of the belt member also affects large the detection error. The detection error occurs in not only the belt member for image formation. The detection error is also a serious problem in a belt member for which a moving quantity per unit time and a moving position at a predetermined time are required to be controlled in high precision, like the belt member for image formation.
In order to increase the precision in the resolution with the encoder according to the conventional method, it is necessary to utilize a main scale having a small lattice constant. In order to maintain the contrast, the main scale and the index scale need to be disposed close to each other.
In other words, the image forming apparatus using the conventional encoder needs to satisfy the following. When the main scale is formed on the surface of the belt or the drum to structure a reflection-type encoder in order to measure a surface moving quantity of the intermediate transfer belt or the transfer paper conveyer belt, it is necessary to provide a certain level of gap between both scales to avoid a vertical move due to ruffling of the belt during the belt running, or a contact between the main scale and the index scale due to an eccentricity of the drum. When the gap between the scales varies or when the main scale is inclined from a normal state, the position of the reflection light incident to the light receiver changes. This has a risk of the occurrence of a measurement error.